The present invention relates to a testing element for testing the positioning and mounting tolerances for fixation of electronic components on printed circuit boards by assembly robots.
The assembly of printed circuit boards and electronic or electric components and component groups by assembly robots may result in inexact positioning and resulting position deviations when placing the components on the printed circuit board because of mechanical, optical, or control-technological effects as well as subjective inputs by the operator. In order to ensure minimal positioning and mounting tolerances for assembly, it is therefore necessary to test the assembly robots in certain intervals.
Under real process conditions the achievable position exactness at the individual assembly points of a robot is subject to stochastic fluctuations the distribution of which cannot be in each case defined by known distribution functions. Thus, basic conditions for a sufficiently precise mathematical prognosis of the distribution of the mounting tolerances across the working area of the assembly robot are not available.
For determining the actual achievable mounting tolerances, it is therefore necessary to detect a large number of measuring points within the working area of the assembly robot by measuring-technological means. Subsequently, with the aid of correction programs, the nominal/actual deviation (variance) at each determined point of the working area can be determined and error correction can be performed.
There is the possibility to determine the processing quality of the assembly robot quantatively as well as qualitatively and to deduce therefrom the remaining service life or the latest point in time at which a new robot must be purchased.
For determining the nominal/actual deviations (variance) for the positioning of SMD components on a circuit board it is customary to employ dummies which are to be mounted under simulated, substantially process-identical conditions.
From German Patent Application 42 27 667 a means for measuring-technological detection of the assembly precision of assembly robots for SMD components is known in which the tolerances of real components is simulated by dummies. An actual printed circuit board is replaced by a testing board made of glass onto which the assembly robot places dummies of SMD components in the form of glass dummies. The single piece, parallelepipedal glass dummies have at their surface a coordinate system and additional markings at the corner points. Additionally, the surface of the glass dummies is provided with a plurality of graphic rasters which simulate the arrangement of pins of a real SMD component. With a corresponding dimensioning of the pin raster, the tolerance range for the photo-optical testing of the mounting precision of the SMD assembly robots can be defined.
The placement of test dummies onto the testing board allows for determination of the stochastically occurring positioning precision at all measuring points of the working area under real assembly conditions of the respective assembly robot.
An advantage of this solution is that the dummies (testing board and glass dummies) can be produced with much greater manufacturing tolerances than real printed circuit boards and components. A further advantage is that due to the minimal manufacturing tolerances the sensing precision of the markings on the testing board and the glass dummies is substantially higher than for real components.
In order to prevent a position change of the glass dummies on the testing board, the parts to be attached to one another are temporarily fixed. For this purpose the surface of the testing board is completely or selectively, by employing a mask, coated with a sprayable adhesive. In an alternative embodiment, the surface of the testing board is provided with a known double-sided adhesive foil. Subsequently, the parallelepipedal glass dummies are placed with their entire bottom surface area onto the coated surface of the glass printed circuit board.
This known method has the disadvantage that due to the non-homogenous properties of the adhesive as well as the flow tendency of the adhesive upon application of the adhesive onto the surface of the testing board it is impossible to provide a uniform, constant adhesive thickness.
Because of the viscosity of the adhesive layer into which the glass dummies upon placement onto the testing board are completely immersed with their bottom surface, after completion of placement flow processes occur which may result in a subsequent position change of the glass dummies on the testing board and thus in a falsification of the measuring results.
Another disadvantage is that the test dummies after completion of measurement and before a further testing series is started, must be completely freed of adhesive remains. Remaining particles of the adhesive and also dirt particles adhered thereto result in a contamination of the testing board which requires a complicated after treatment in order to prevent mechanical damage at the glass surface of the dummies.
The relatively expensive glass dummies furthermore are sensitive to impact and surface wear. This known method for position fixation of the glass dummies therefore does not allow for a permanent process monitoring of assembly robots because of the aforementioned technological flaws.
It is an object of the invention to eliminate the disadvantages of the prior art.